1. Field of the Invention
The present invention relates to a label printer, and particularly to a label feed control system for a label printer which issues a label after data from the weighing unit has been printed on the label stuck on the paper base ribbon.
2. Description of the Prior Art
An example of the conventional label feed control system will be described with reference to FIGS. 1 through 4. On a paper base ribbon 1 there are stuck labels 2 of a certain size in a constant interval, and such a ribbon 1 is wound on a label supply reel 3. The ribbon 1 is transported to a ribbon take-up reel 8 through a printer 4, a separator 5 on which a label is peeled off the base ribbon, and a feed roller 7 driven by a motor 6. On the upstream side of the printer 4 there is provided a photoelectric label position detector 9, and a label detector 10 for detecting a peeled-off label 2 is provided on the front of the separator 5.
The weighing unit 11, the printer 4 and a keyboard 12 are connected to a CPU 13, which is further connected to an I/O port 14. The label detector 10 is connected through a label detection amplifier 15 to the I/O port 14. Also connected to the I/O port 14 is a feed controller 16, which is further connected to the motor 6 and to the position detector 9 through a position detection amplifier 18 having a variable resistor 17.
The label detector 10 produces a high D-signal when a label is absent, and when an operation command, i.e. an A-signal, is issued with the signal D being high, the feed controller 16 produces a high C-signal to activate the motor 6 so that the ribbon 1 is fed. The signal C is also delivered to the CPU 13 via the I/O port 14 so as to interlock other operations during the transportation of the ribbon. As the result of transporting the ribbon 1, a label 2 is peeled off the ribbon 1, projecting over the label detector 10 to cause its output signal D to become low. When the position detector 9 detects the label position and produces a B-signal, as will be described shortly, with the signal D being low, the signal C from the feed controller 16 goes low to stop the motor 6 and also to release the inhibited commands in the CPU 13.
The position detector 9 operates by sensing the transmissivity of the base ribbon 1 and the label 2. There are three cases in the degree of transmissivity as shown in FIG. 2-b: (a) with the base ribbon 1 alone, (b) with the label 2 on the base ribbon 1, and (c) with the label 2 having a printed portion 20 stuck on the base ribbon 1. The signal B is produced at the position where the base ribbon 1 alone-exists. This system is based on the detection of the difference of light transmitted through the base ribbon 1 alone and the overlap of the base ribbon 1 and the label 2, and has the following problems. A label 2 having a high light transmissivity results in a very small difference in the light level, requiring disadvantageously a very high accuracy of detection. If the label 2 has a printed portion 20 as mentioned above for the shop name and the like, the lower transmissivity of this portion creates a large contrast relative to remaining portions of the label, resulting possibly in a failure of detection. Moreover, it is irksome to adjust the sensing level by the variable resistor 17 each time the thickness of the base ribbon 1 is changed. In addition, the position detector 9 needs to be repositioned for each label size, and since the signal B from the position sensor 9 also serves as the operational reference for the printer 4, several labels are wasted for test printing before the best set position is determined.
The applicant has proposed an arrangement for positioning the label accurately by detecting it reliably. In such arrangement, instead of the position detector 9 in FIG. 1 the label detector 10 functions to detect the front edge of the label 2 and also the presence of a label, so that the label is fed for a certain length in response to the detection of the front edge. However, since the detector 10 is located in front of the separator 5, if the operator put his fingers into the detector 10 to pick the label 2 during printing, the detector 10 responds to the fingers before it correctly detects the front edge of the label 2. Thus the label is fed for a certain length from that point abnormally, resulting in a shift in the print position when the label is printed by the printer 4, and the label is wasted.
Furthermore, if the label detector 10 fails to detect the label, holding the system in a virtual label absent state, label transportation is not controlled properly and labels are wasted.
The present invention is contemplated to overcome these deficiencies while utilizing the advantages of the front edge detection system. Accordingly, an object of the invention is to provide a label feed control system which minimizes the label positioning error when the label detector creates an erroneous signal or fails to operate.